Certain processes for producing titanium dioxide generally rely on the use of chlorine, hydrochloric acid or sulfuric acid. As such, a waste stream of by-product hydrochloric acid or by-product sulfuric acid is produced. A by-product hydrochloric acid contains a variety of contaminants including colloidal silica, colloidal titanium dioxide, titanium oxychloride, and lesser amounts of other trace and heavy metal impurities. Currently, there is approximately 105,959,800 gallons (523,000 short tons) of by-product hydrochloric acid produced annually in the United States and Canada alone. Typically, this by-product acid is disposed of as waste due to the great amounts of contaminants preventing further use of the by-product acid.
An example process for manufacturing titanium dioxide consists of heating an ilmenite ore or a rutile titanium oxide containing sand ore, and coke to a controlled roasting temperature of approximately 1000° C. A stream or current of chlorine gas is passed into the heated carbon ore, at which point titanium tetrachloride, silicon tetrachloride, iron chloride, cadmium chloride and other trace inorganic chloride gases are formed. The lower volatile inorganic chloride gases with any of the other unreacted beginning materials are removed by condensation. The titanium tetrachloride and a very small fraction of some of the lower volatile compounds are oxidized with oxygen at 1400° C. to 1600° C. to form SiO2 and crystals of rutile TiO2 (the desired titanium dioxide) product of a required size. The titanium dioxide crystals are removed in the vapor phase, and the remaining vapors are quenched with water. The quenching produces by-product hydrochloric acid that contains contaminants of titanium dioxide, titanium oxychloride, silica and small amounts of acid soluble chlorides, e.g. iron chloride.
These contaminants generally prohibit the re-use of the hydrochloric acid in other processes. For example, the concentration levels of contaminants such as silica and titanium dioxide in the hydrochloric acid stream render the acid unusable for most steel pickling processes and chemical manufacturing processes. Instead, the acid streams are disposed as waste, stored indefinitely, and/or possibly subjected to a variety of reclamation processes that can be time-intensive and costly. Often, the waste streams are neutralized, such as with caustic soda or hydrated lime, to form a brine solution for disposal.